1. Field of the Invention
The present invention relates to a fixing device applied to an image forming apparatus such as a copier, a facsimile apparatus or a printer and an image forming apparatus provided with such a fixing device and, particularly to a fixing device for fixing a toner image to a transfer material by induction heating and an image forming apparatus provided with such a fixing device.
2. Description of the Related Art
An image forming apparatus is constructed such that a beam based on image information is emitted to the outer circumferential surface of a rotating photosensitive drum and toner as developer is supplied to an electrostatic latent image thus formed on the outer circumferential surface to form a toner image. The toner image formed on the outer circumferential surface of the photosensitive drum is transferred to a conveyed sheet as a transfer material and fixed to the sheet by heating in a fixing device. The sheet having the toner image fixed thereto is discharged to the outside from an apparatus main body.
The fixing device normally includes a fixing roller heated at a high temperature and a pressure roller opposed to the fixing roller such that the outer circumferential surface thereof is in contact with that of the fixing roller, and a fixing operation is performed by feeding the sheet to a nip portion between these two rollers. A halogen lamp built in the fixing roller has been conventionally used as a heating source for the fixing roller, but it has problems of poor heat efficiency and lacking swiftness due to a long time required to warm up (time required to be sufficiently heated). It has been attempted to reduce the heat capacity of the fixing roller or to thin the fixing roller in order to solve such problems, but there is a limit to it.
Accordingly, in recent years, attention has been paid to a fixing device of the induction heating type for heating a fixing roller by induction heating as disclosed in Japanese Unexamined Patent Publication No. H09-127810. This fixing device of the induction heating type has a fixing roller comprised of a hollow metal roller having good heat conductivity and being nonmagnetic and a magnetic metal thin layer formed on the outer circumferential surface of this hollow metal roller and made of magnetic metal. An induction coil is provided inside such a fixing roller, and the fixing roller is heated by Joule heat produced by exciting the induction coil to produce an eddy current in the magnetic metal thin layer.
By adopting such a fixing device of the induction heating type, the temperature rising rate of the fixing roller is remarkably speeded up as compared to fixing devices of the conventional halogen lamp type, wherefore the warm-up period of the fixing device can be speeded up. However, this has raised a new problem of overheating the fixing roller because the temperature rises too quickly. In order to solve such a problem, a feedback control is executed to detect the temperature of the fixing roller by means of a temperature sensor such as a thermistor or a thermostat and to shut off the supply of power to the induction coil if the detected temperature becomes equal to or higher than a preset temperature, but there still exists an inconvenience that the output of a detection signal from such a temperature sensor may not be able to follow the temperature rise by induction heating due to a time lag, resulting in the overheating of the fixing roller.
Further, as the fixing roller is thinned, there is a tendency to make it more difficult to smoothly transfer heat along longitudinal direction. Thus, if sheets smaller than a heated range are successively fed, heat tends to be trapped at the opposite ends of the heated range where sheets pass at a low frequency. If a fixing operation is applied to a wide sheet in this state, there is an inconvenience of causing an image error such as a so-called offset phenomenon in which a toner image on this sheet is fused and adhered to the fixing roller to be transferred to a next sheet.
In order to solve such an inconvenience, Japanese Unexamined Patent Publication No. 2004-151470 discloses that a fixing roller 90 is comprised of a tubular temperature-sensitive metal layer 91 made of temperature-sensitive metal and a nonmagnetic metal layer concentrically placed on the outer circumferential surface of the temperature-sensitive metal layer 91 and made of nonmagnetic metal, and an induction coil 93 for creating a magnetism is arranged in the tubular temperature-sensitive metal layer 91 as shown in FIGS. 11A and 11B. In such a fixing roller 90, thickness t (m) of the temperature-sensitive metal layer 91 is set to satisfy the following inequality:
      503    ×                  (                  ρ          /                      (                          μ              ⁢                                                          ⁢              s              ×              f                        )                          )              <  t  <      503    ×                  (                  ρ          ⁡                      (                          1              ×              f                        )                          )            (where ρ: resistivity (Ω·m) of the temperature-sensitive metal, f: frequency (Hz) of a power supply for the induction coil, μs: specific permeability at a temperature equal to or below a Curie temperature of the temperature-sensitive metal)
In this inequality, “503√{square root over ( )}(ρ/(μs×f))” represents depth of magnetic permeation when the temperature of the temperature-sensitive metal layer 91 is equal to or below the Curie temperature (transition temperature), and “503√{square root over ( )}(ρ/(1×f))” represents depth of magnetic permeation when the temperature of the temperature-sensitive metal layer 91 is above the Curie temperature.
By adopting the thus constructed fixing roller 90, the depth of magnetic permeation is smaller than the thickness of the temperature-sensitive metal layer 91 when the temperature of the temperature-sensitive metal layer 91 is equal to or below the Curie temperature. Thus, a load (electric resistance) caused by a resulting eddy current increases (i.e. an excess current density increases to increase the load due to the flow of a current in a narrow region), and the magnetism flows along longitudinal direction in the temperature-sensitive metal layer 91 having a large electric resistance as shown by arrows in FIG. 11A, whereby the temperature-sensitive metal layer 91 is quickly heated by a large amount of heat (Joule heat) produced by the load resulting from the eddy current.
If the temperature of the temperature-sensitive metal layer 91 exceeds the Curie temperature due to this heating, the depth of magnetic permeation becomes larger than the thickness of the temperature-sensitive metal layer 91. Thus, the magnetism reaches the nonmagnetic metal layer 92 having a smaller resistivity than the temperature-sensitive metal layer 91, and travels toward a direction of a center axis in the nonmagnetic metal layer 92 as shown in FIG. 11B, whereby the amount of produced heat is suppressed to suppress the overheating of the fixing roller 90.
Accordingly, if such a fixing roller 90 is adopted, there is an effect of being able to prevent the overheating of the fixing roller 90 without executing a control to detect the temperature of the fixing roller 90 by means of the temperature sensor such as a thermistor or a thermostat and to suppress the temperature of the fixing roller 90 (i.e. without any time lag caused by an output delay of the detection signal in the case of such a control).
In the fixing roller 90 disclosed in Japanese Unexamined Patent Publication No. 2004-151470, an alloy of iron (Fe) and nickel (Ni) is used for the temperature-sensitive metal layer 91 and aluminum (Al) is used for the nonmagnetic metal layer 92.
However, in the fixing device disclosed in Japanese Unexamined Patent Publication No. 2004-151470, heating efficiency by induction heating (particularly, temperature rising rate during the warm-up period) depends only on the characteristics of the temperature-sensitive metal (specifically, values of the specific resistivity and the permeability at a temperature equal to or below the Curie temperature). Therefore, there remains a problem of being unable to further improve the heating efficiency.